Oxide superconductors exhibit superconductivity at relatively high temperatures in comparison to their traditional metallic counterparts. However, the oxide superconductors, being ceramics, are generally brittle and are difficult to process and manipulate. In contrast, composites of oxide superconductors supported by metal matrices, such as metal sheaths, have mechanical and electrical properties that are improved relative to the oxide superconductors alone. The metal sheath, matrix, or container, which holds the oxide superconductor powder prior to and during processing is typically composed of a noble metal such as silver or a silver alloy.
Oxide dispersion strengthened (ODS) silver alloys have been used as matrix materials, where the oxide dispersion increases the strength and hardness of the matrix. The use of silver and silver alloys in the production of silver-sheathed superconductor composites (silver-superconductor composites) is described, for example, in Lay, U.S. Pat. No. 5,384,307, Yamamoto, et al., U.S. Pat. No. 5,232,906, Flukiger, U.S. Pat. No. 5,232,906, and U.S. Ser. No. 08/626,130 filed Apr. 5, 1996 and entitled "Oxygen Dispersion Hardened Silver Sheathed Superconductor Composites," each of which is incorporated herein by reference.
Silver-sheathed superconductor composites require the fabrication of thick walled and large diameter silver tube stock (used as the metal sheath) for processing, however, available silver tube stock has primarily been prepared to the specifications of the jewelry industry and has relatively thin walls and small diameters. Larger articles of silver are available as cast ingots having diameters up to about 100 millimeters. The properties of the silver-superconductor composites are influenced by the starting grain size of the silver.
Cast silver is not well-suited for use in silver-superconductor composites due to the large average grain size and possible high level of porosity in the silver or silver alloy castings. In making these castings, only small temperature gradients exist in the solidifying metal due to the high thermal conductivity of the silver, which results in very large grains in the castings. The grain sizes typically can be on the order of 300 micrometers to a few millimeters in diameter. In addition, cast silver can have porosity due to bubble formation during cooling of the silver when the melt is not properly degassed. The high level of porosity occurs because the solubility of gases in solid silver is much lower than the solubility of gases in liquid silver. The large grain sizes and porosity lead to difficulties in silver-superconductor composite processing steps.
Presently, the process for preparing silver tube stock suitable for use in silver-superconductor composites is to cast molten silver into a tube up to about 50-75 millimeters in diameter. The cast silver tube is then drawn to the desired inner diameter (ID) or outer diameter (OD). Drawing of the cast silver tube to reduce its OD by a nominal factor of about 2 (an OD on the order of 25-35 millimeters) produces a number of flaws, such as folds, in the product that relate to the large initial grain size and high porosity of the cast silver tube. First, the large grains can exaggerate the formation of steps and folds on the inner surfaces of the drawn silver or silver alloy tube. These may extend up to 1 millimeter below the surfaces of the drawn tube. The steps and folds are typically formed on the inner surface due to the unsupported collapse of the inner diameter of the tube during drawing. Drawing a tube with insufficient mandrel support on the inside of tube can may cause folds in fine grains as well. Once created, the folds can only be removed by machining the effected portion of the surface, resulting in a yield loss of material. Second, the large grains can result in non-uniform deformation and formation of localized cracks or tears on the surface of the tube during processing. Cracks or tears can form either on the inner or outer surface of the tube during drawing. During drawing, junctions where three grain boundaries meet (grain boundary triple points) can open in early stages of drawing. These openings becomes lenticular during further drawing steps and eventually becomes an axial "split" in the silver tube of the monofilament The split remains in the monofilament, even though the drawing work eventually refines the silver grains to smaller grain sizes. The split can result in bridging between filaments in a multifilamentary composite configuration. Preferably, filaments are unbridged. Third, porosity in the cast silver material is not healed in the processing since the drawing forces are primarily tensile rather than compressive and are applied at or near room temperature.
The tube casting and tube drawing approaches to the production of silver or silver alloy tube stock needed to make silver-sheathed superconductor composites can be expensive. Tube casting is more expensive than cylindrical billet casting. The tube drawing process has yield loss on the ends of each drawn length. Tube drawn product must be post-machined to remove sizable surface defects such as the folds that can be up to 1 millimeter deep.
The requirements for cast and drawn silver or silver alloy monofilament processability, final tape performance, and processing costs indicate that alternative routes to silver or silver alloy tube stock are desirable. Moreover, defects and failures encountered in processed silver-superconductor composites suggest that silver or silver alloy tube quality needs to be improved. In addition, larger diameter, thicker wall silver or silver alloy tube stock than that typically used in the jewelry industry that is suitable for use in composite processing is needed.
One family of oxide superconductors includes bismuth-strontium-calcium-copper-oxide (BSCCO) compositions such as Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 (BSCCO-2212) and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (BSCCO-2223). The BSCCO superconductors include compositions where bismuth is partially substituted by dopants such as lead (that is, (Bi,Pb)SCCO). Other families of oxide superconductors include yttrium-barium-copper-oxide (YBCO) compositions, such as YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO-123), and thallium-barium-calcium-copper-oxide compositions.
The oxide superconductor-silver composites can be prepared in elongated forms such as wires and tapes by processes, such as the well-known powder-in-tube (PIT) process, that typically include a number of stages. In the PIT process, first, a powder of a precursor to a superconductor is prepared. The precursor can be a single material or a mixture of materials. Second, a silver or silver alloy container (for example, a tube, billet or grooved sheet) is filled with the precursor powder. The silver container serves as a matrix for constraining the superconductor. Third, the filled container is deformed in one or more iterations to reduce the cross-sectional area of the container in a draft reduction step. A number of filled containers (filaments) can be combined and surrounded by another silver or silver alloy matrix to form a multifilament article. Finally, the material is subjected to one or more deformation and annealing cycles which together form and sinter the oxide superconductor. This thermomechanical processing helps precursor grains align and grow to form a textured superconductor article, which is predominantly composed of one phase and has a high critical current density (J.sub.c).
If the precursor powder is composed of one or more oxides, the process is known more specifically as oxide-powder-in-tube (OPIT) processing. If the precursor powder is composed of elemental metal alloys, the process is known more specifically as metal-powder-in-tube (MPIT) processing. See, for example, Yamamoto, et al., U.S. Pat. No. 5,232,906, Otto, et al., "Progress toward a long length metallic precursor process for multifilament Bi-2223 composite superconductors", IEEE Transactions on Appl. Supercon., Vol. 5, No. 2 (January 1995), pp, 1154-1157, Yurek, et al., U.S. Pat. No. 4,826,808, Yurek, et al., U.S. Pat. No. 5,189,009, Gao et al., Superconducting Science and Technology, Vol. 5, 1992, pp. 318-326, and Sandhage, et al., "The oxide-powder-in-tube method for producing high current density BSCCO superconductors", Journal of Metals, Vol. 43, No. 3, 1991, pp. 21-25, each of which is incorporated herein by reference.